Abstract : Six dogs under morphine-pentobarbital anesthesia were exposed to forward accelerations of 2, 4 and 6G for one minute and 6G for three minutes while in the horizontal, 15 degrees head-up and 15 degrees head-down positions breathing room air. Exposures to 6G were repeated breathing 99.6% oxygen. Oxygen saturation and opacity at 800 millimicrons of blood were recorded continuously by cuvette oximeters. Pulmonary arterial-venous shunting was estimated from blood oxygen saturations. No systematic changes in femoral artery oxygen saturation occurred at 2G while a small average decrease was observed at 4G (4%). Decreases occurred at 6G averaging 11 (5-17) per cent at the end of the 60-second exposure. Return to control (1G) values was nearly complete 50 seconds after the exposure. Oxygen inhalation delayed but did not prevent the desaturation. These decreases are believed due to pulmonary arterial-venous shunting. The average increase in pulmonary arterialvenous shunt over 1G values estimated at the end of 60second exposures to 6G when breathing air, was 17 (11-31) per cent. Values for shunts at 6G, when breathing oxygen, were similar.